1. Field of the Invention
The present invention relates to a method for cooling high temperature reduced iron produced by a direct reduction process in a cooling zone integrated with a direct reduction furnace, such as a shaft furnace, which method utilizes the exhaust gas of the reducing furnace as the cooling gas with great operational and economical advantages.
In recent years, so-called direct iron-making processes, which reduce iron ores with gases or carbonaceous materials, have been used more and more in commercial practice.
These direct processes have certain advantages over the blast furnace process, which is presently the main source for iron and steel making, in that energy to be used can be more freely selected, less capital cost is required for the apparatus, thus they are more suitable for a small scale of production, and a high degree of technics as required by the blast furnace process is not required. Due to these advantages, the general tendency is that the direct iron-making processes will produce an increasing amount of iron on a commercial scale.
For reduction of iron ores in the direct iron-making processes, the iron ores are heated at a temperature not lower than 700.degree. C. with solid carbonaceous materials, such as coal and coke; or reducing gases, such as natural gas, petroleum fuel and modified coke-oven gas; or coal gas. In order to remove the gangue component and refine the reduced iron in a melting furnace, the reduced iron is transported to the melting furnace. If the melting furnace is close to the reducing furnace, it is possible to transport and discharge the hot reduced iron to the melting furnace as "hot charge."
In most cases, however, the hot reduced iron is once cooled and then transported to the melting furnace. The cooling has been done by briquetting the hot reduced iron and leaving the briquetts in the air, or by means of a reducing gas or a neutral gas.
In recent years, a high-pressure direct reducing furnace with an increased furnace gas pressure has been in commercial use for improving the production efficiency of a direct reducing furnace. In this type of a direct reduction furnace, the increased furnace gas pressure increases the reduction rate of the iron ores, and makes it possible to increase the amount of reducing gas for improved productivity without increasing the gas flow velocity through the furnace, hence being free from various troubles, such as hangings and fluidization. For adjustment of the furnace gas pressure in a high-pressure direct reduction furnace, usually a gas pressure adjusting device is provided at the gas exhaust portion, so as to maintain a furnace gas pressure normally in the range from 1.5 to 5 kg/cm.sup.2. The desired effect of the high pressure operation can be expected when the furnace gas pressure is 1.5 kg/cm.sup.2 or higher, and for a commercial operation it is preferable to maintain the furnace gas pressure at 2 kg/cm.sup.2 or higher. On the other hand, with a furnace gas pressure higher than 5 kg/cm.sup.2, the reduction rate is not substantially improved and it becomes difficult to control the gas pressure.
For cooling the high-temperature reduced iron produced in a high-pressure direct reducing furnace, it is necessary to cool it in a cooling zone communicating with the reducing furnace under a high-pressure condition, because the temperature within the furnace is at lowest 700.degree. C. or higher and it is difficult to seal the pressure under the high temperature condition, and thus it is impossible to take out the reduced iron from the furnace at the high temperature by reducing the furnace gas pressure to the atmospheric pressure.
In the case where the reduced iron is cooled in the cooling zone integrated with the high-pressure direct reducing furnace, when a reducing gas containing CO is used as the cooling gas, carbon precipitation is caused in the reduced iron layer in the furnace, resulting in various troubles, such as hangings, and ultimately prohibiting the operation of the furnace as a whole. This carbon precipitation is more remarkable as the furnace pressure increases.
More particularly, this carbon precipitation is caused by the following reaction, and when metallic iron is present in the reaction system, it acts as a catalyst to promote the carbon precipitation and the reaction is more promoted at a higher pressure. EQU 2CO.fwdarw.CO.sub.2 +C
The above reaction is remarkable at a relatively low temperature between 400.degree. and 600.degree. C., and when the reduced iron is cooled from a temperature of 700.degree. C. or higher, the cooling passes through the above temperature range, so that some carbon precipitation is unavoidable.
In a high-pressure direct reduction furnace, the cooling is done under an elevated pressure and the above reaction is sharply promoted, so that it is impossible to cool the reduced iron with the reducing gas composition used as a cooling gas in a normal-pressure reducing furnace.
Also when the reduced iron is cooled in a high-pressure direct reduction furnace, methane formation is caused by the following reactions between CO and H.sub.2 in the cooling gas. EQU CO+3H.sub.2 .fwdarw.CH.sub.4 +H.sub.2 O EQU 2CO+2H.sub.2 .fwdarw.CH.sub.4 +CO.sub.2
Both of the above reactions are exothermic, so that when the above methane forming reaction is caused, the cooling efficiency is markedly lowered. These reactions, just as the carbon precipitation reaction, are also promoted as the furnace pressure is increased.
Now when the reduced iron is treated in a subsequent step, such as in an electric furnace, there is one considerable problem to be solved. The reduced iron is reoxidized during its storage in the air after cooling or during its transportation. The reduced iron usually has a high porosity (30 to 70%) and is easily permeated by the air, and its reoxidation is promoted by the presence of water. The reduced iron, when reoxidized, is not only degraded in its commercial value due to the lowered metal value, but also is very dangerous because it burns due to the exothermic reaction of rapid reoxidation and it may cause fires.